iwi discarding oversized packets while mtu=1500 for src/dst

[Sten Daniel Sørsdal]

Sam Leffler wrote:
> Hans Nieser wrote:
>> Sam Leffler wrote:
>>> Hans Nieser wrote:
>>>
>>>> root at aphax-laptop:~# uname -a
>>>> FreeBSD aphax-laptop.lan 6.1-RELEASE FreeBSD 6.1-RELEASE #0: Thu May 11
>>>> 07:17:09 CEST 2006
>>>> root at aphax-laptop.nieser.local:/usr/obj/usr/src/sys/APHAX-LAPTOP  i386
>>> Are you running the iwi driver that came with 6.1-release?  If so it has
>>> numerous problems that have been fixed in 6-STABLE and HEAD.  I'm not
>>> sure how best to update your system except by going to 6-STABLE via a
>>> src upgrade.
>> Today I have upgraded my laptop to 6-STABLE, but unfortunately the
>> problems remain. I don't even know wether iwi is to blame, because I can't
>> figure out wether perhaps xl on my server really is sending out too large
>> packets.
>>
>> That is, so far I've been assuming that an mtu of 1500 may not mean that
>> there's exactly 1500 bytes going over the wire (overhead of protocols to
>> which the mtu doesn't apply or something, I dunno, I'm no networking
>> expert obviously :), because xl is definitely sending out packets of 1518
>> bytes. Which iwi on my laptop doesn't like, but the NIC in my desktop
>> machine (which runs Linux) has no problem with. Maybe someone can tell me
>> which of the machines is in error here, at least I'd know what to blame :(
>>
>> The fact that my Linux box doesn't discard these packages coming from my
>> server made me suspicious of iwi initially, but maybe iwi is doing nothing
>> wrong and my Linux box is simply willing to accept these oversized packets.
>>
> 
> I can't speak to linux but an mtu of 1500 will cause a 1518 byte packet
> to be discarded.  I don't recall what your problem was but if the iwi
> driver is receiving the frame and passing it up only to be discarded by
> the 802.3 layer then you've received a frame that's too large and you
> should look at the sender side for why it's being generated.  If you
> don't want to do that you can probably just up the mtu on iwi and let
> the frame through.
> 

This does sound awfully lot like the driver doesn't strip the 4 byte
checksum of a basic 802.[23] ethernet frame. My initial thought was that
it was a vlan frame but the frame protocol indicates it's an IP packet.
Perhaps it's a frame that slips through the AP's checks on it's way out
to the wireless?

-- 
Sten Daniel Sørsdal